I was with 3 a few years ago, the coverage in my area (inner south east melb) was pretty bad. Calls would drop out all the time and looking at the screen to see 'no reception' was a fairly common occurrence. The customer service is pretty hopeless but you'll find that with any telco provider I guess. When I called to end my contract they kept transferring me to various departments trying to get to me stay. I know they're just doing their job but just cancel my account already! I've now switched to Optus and the coverage is much better but apart from that I don't find the 2 be any different. Not sure whereabouts you are in Melbourne, I suspect if you are closer to the CBD the coverage would be considerably better.

HTH